The denticle surface of thresher shark tails: Three‐dimensional structure and comparison to other pelagic species

Abstract

Shark skin denticles (scales) are diverse in morphology both among species and across the body of single individuals, although the function of this diversity is poorly understood. The extremely elongate and highly flexible tail of thresher sharks provides an opportunity to characterize gradients in denticle surface characteristics along the length of the tail and assess correlations between denticle morphology and tail kinematics. We measured denticle morphology on the caudal fin of three mature and two embryo common thresher sharks (Alopias vulpinus), and we compared thresher tail denticles to those of eleven other shark species. Using surface profilometry, we quantified 3D-denticle patterning and texture along the tail of threshers (27 regions in adults, and 16 regions in embryos). We report that tails of thresher embryos have a membrane that covers the denticles and reduces surface roughness. In mature thresher tails, surfaces have an average roughness of 5.6 μm which is smoother than some other pelagic shark species, but similar in roughness to blacktip, porbeagle, and bonnethead shark tails. There is no gradient down the tail in roughness for the middle or trailing edge regions and hence no correlation with kinematic amplitude or inferred magnitude of flow separation along the tail during locomotion. Along the length of the tail there is a leading-to-trailing-edge gradient with larger leading edge denticles that lack ridges (average roughness = 9.6 μm), and smaller trailing edge denticles with 5 ridges (average roughness = 5.7 μm). Thresher shark tails have many missing denticles visible as gaps in the surface, and we present evidence that these denticles are being replaced by new denticles that emerge from the skin below.

Dermal Denticle Diversity in Sharks: Novel Patterns on the Interbranchial Skin

“…For example, in species like the smooth dogfish and thresher shark, denticles from the leading edge of the tail and fins tend to be flattened, more rounded, and have reduced ridges compared with the denticles on the trailing edges, which are triangular with multiple ridges ( Ankhelyi et al. 2018 ; Popp et al. 2020 ; Reif 1985b ).…”

Aerodynamic Characteristics of Shark Scale-Based Vortex Generators upon Symmetrical Airfoil

“…To overcome the performance degradation caused by flow separation, researchers [1][2][3][4] have recently focused their attention on biomimetic devices and systems related to aerodynamics and flow control technologies. The present paper primarily focuses on resolving the flow separation over airfoils, as they have become omnipresent.…”

“…Flow control is the method of manipulating or creating the desired boundary layer profile over the object of interest, increasing the lift, decreasing the drag, or delaying the flow separation. Several researchers have stated that flow control over an airfoil is necessary to ensure a better performance [1][2][3][4][5]. Therefore, it becomes clear that flow control technologies have emerged as a promising solution to overcome the poor aerodynamic performance caused by the boundary layer separation on aerofoils' upper surface.…”

Sharks as exfoliators: widespread chafing between marine organisms suggests an unexplored ecological role

“…1, Videos S1, S2; Appendix S1: Table S1). We hypothesize that fish chafe in specific regions on the shark's body because the dermal denticles in these areas are more effective in removing parasites due to variation in dermal denticle morphology across the shark's body (Ankhelyi et al 2018, Popp et al 2020). However, it could also be because those regions are associated with the lowest predation risk.…”

“…This leads to the broader ecological question as to why this phenomenon does not appear to occur in terrestrial ecosystems. Moreover, many terrestrial animals have rough or scaly skin, but there is no terrestrial equivalent for dermal denticles, whose sandpaper-like texture and posterior orientation appear to evolve primarily for hydrodynamic efficiency in a dense, viscous medium (Popp et al 2020). This unique functional morphology could be why chafing behavior is found exclusively in aquatic systems.…”